A man went on trial for the murder of Pc Keith Blakelock, nearly 30 years after he was attacked by a mob yelling "kill the pig" during the first Tottenham riots.

Nicky Jacobs, 45, is accused of stabbing Pc Keith Blakelock, 40, as the officer tried to protect firefighters tackling a blaze at the height of the unrest in on the Broadwater Farm estate in north London in 1985.

A jury at the Old Bailey was told this was the second trial over Pc Blakelock's murder, the first set of convictions being quashed on appeal.

A fresh investigation into the attack led to the decision to give immunity to some people who admitted kicking and punching the officer, with police focusing on those who attacked with weapons, the court was told.

Rather than facing murder charges, some rioters will be giving evidence under pseudonyms because only those in the "inner circle" of the crowd around the officer were close enough to see.

Opening the prosecution at the Old Bailey, Richard Whittam QC told jurors about the police "dilemma" and warned them to treat the witnesses' evidence with care.

He said they may "strongly disapprove" of their characters, with some having criminal convictions, and drink and drug problems. He also disclosed that some witnesses had been paid rewards for their co-operation.

He told the jury: "Your task is likely to be centred on deciding whether or not you can be sure that these witnesses are telling the truth when they tell you that Nicky Jacobs took part in the attack with a bladed article, acting with others."

Mr Whittam described to the court how the riots erupted on the night of October 6 1985, the day after a suspect's mother, Cynthia Jarrett, had a heart attack and died during a police search of her Tottenham home.

Jacobs was 16, almost 17, at the time of the riot, which followed weeks of tension and concerns that "individuals were planning public disturbances" in the borough of Haringey.

The riots were more "sinister" than the later ones in 2011, and at least some rioters were intent on killing a police officer, Mr Whittam said.

He said: "At least some of the rioters in 1985 appeared to have as their target the death of a police officer.

"Whether that was their primary objective is not something that you will have to decide. The fact is that one police officer was killed and another very seriously injured."

Pc Blakelock was among a group of uniformed officers sent out without cover to protect firemen putting out blazes.

They came across a "very large group" of rioters, many armed with an assortment of weapons, the court heard.

"Very heavily outnumbered and fearful they may become trapped both the police and the firefighters were forced to retreat. Outside the flats, as they ran for safety, Pc Blakelock and Pc Richard Coombes went to ground and were set upon to shouts of 'kill the pig' and the like."

"Pc Coombes was very fortunate to survive. Pc Blakelock did not. The attack on him was without mercy. In the ferocious attack his helmet came off.

"He was beaten and stabbed to death before his colleagues were able to force the attackers away.

"Pc Blakelock suffered something in excess of 40 stab type injuries and there appears to have been an attempt made to decapitate him."

Mr Whittam told the jury that the allegation against Jacobs was that "he was armed with a bladed weapon and he used it as part of the joint attack" on Pc Blakelock.

"There is no dispute that Pc Blakelock was murdered," he said. "There is no dispute that Nicky Jacobs was involved in the public disorder that night, as were some of the witnesses."

The jury of seven men and five women were given details about the two investigations into the murder prior to the current one.

The first inquiry in 1985 resulted in three juveniles and three adults being charged for murder.

Although the case against the juveniles did not proceed, all three adults were convicted in March 1987.

Their convictions were quashed by the Court of Appeal in 1991, leading to a new investigation in 1992/93.

The prosecutor said that the new inquiry faced a dilemma in trying to get witness accounts of the attack on Pc Blakelock.

While they needed eye witnesses close enough to see clearly what happened, some of them may have taken part themselves, ordinarily making them liable for prosecution.

This led the police and Crown Prosecution Service to a ''highly unusual'' decision - to pursue those who had weapons as suspects and regard unarmed attackers who punched and kicked as witnesses.

The prosecutor told the court that in 1993: ''An agreement was reached outlining a form of conditional immunity from prosecution for some of those involved in the attack.''

During the second investigation, witnesses had also been paid rewards by the police for their "willingness to give evidence and cooperation with that inquiry", Mr Whittam said.

"Since then, under the current investigation, any payments made have been subject to a strict authorisation procedure and are limited," he said.

The third investigation was launched in 2000 through a new system for reviewing unsolved murders.